Photo-acoustic technology applied to ppb level NO₂ detection by using low power blue diode laser

Abstract

Photo-acoustic technology based on a low power blue diode laser for measuring the ppb level NO<sub>2</sub> is presented in this paper. A low-cost NO<sub>2</sub> measurement system based on traditional photo-acoustic technology is established. The 405 nm blue diode laser with an external modulation is used as a light source. The central wavelength of the laser is 403.56 nm, the half-peak full width is 0.84 nm, and the power is 65.3 mW. The effective absorption cross section of NO<sub>2</sub> is obtained, and the interference of the water vapor and other trace gasisinvestigated. The resonant frequency is tested to be 1.35 kHz by frequency scanning fitting. An internally polished and coated poly tetra fluoroethylene aluminum cylindrical cavity is used as a photo-acoustic resonator (the inner diameter is 8 mm and the length is 120 mm). The influence factors caused by cavity parameters, optical windows and power supply are studied. The system is optimized to reduce background noise and improve signal-to-noise ratio. Then the noise signal is dropped to 0.02 <inline-formula><tex-math id="M4">\begin{document}${\text{μV}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182262_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182262_M4.png"/></alternatives></inline-formula>. An additional buffer chamber is integrated on the original buffer chamber to form a two-level buffer. The two-stage buffer structure significantly suppresses the effects of airflow noise and improves the system stability. The slope of the calibration curve of the system after linear fitting is 0.016 <inline-formula><tex-math id="M5">\begin{document}${\text{μV/ppb}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182262_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182262_M5.png"/></alternatives></inline-formula>, and R<sup>2</sup> is 0.998. The NO<sub>2</sub> detection limit of system is 2 ppb (3<inline-formula><tex-math id="M6">\begin{document}$\sigma$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182262_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182262_M6.png"/></alternatives></inline-formula>) with an average time of 60 s. To verify the results of the system, a diode laser cavity ring-down spectroscopy system (CRDS system, using a 409 nm the diode laser, with a system detection limit of 6.6 × 10<sup>–1</sup>) is used to measure ambient NO<sub>2</sub> simultaneouslyon Lake Dong-Pu in western Hefei, Anhui Province, China. During the experiment, the measured NO<sub>2</sub> concentration ranges from 8 to 30 ppb, with an average concentration of 20.8 ppb. The results of two systems have good consistency:alinear fitting slope of 0.94 ± 0.009, an intercept of 1.89 ± 0.18 and acorrelation coefficient of 0.87. The experimental results show that the system can realize the low-cost on-line detection of the ppb level NO<sub>2</sub>, and it can also be used for the real-time detection of NO<sub>2</sub> concentration field.

Miniaturized methane detection system based on photoacoustic spectroscopy

“…The maximum modulation frequency is lower than 100 Hz, which is vulnerable to the interference of low‐frequency signals in the system 23 . For resonant PAC, the sonic waves can form standing waves in the cell and generate resonant amplified signals, 24 which usually be employed in the PAS sensor.…”

Design of NO2 photoacoustic sensor with high reflective mirror based on low power blue diode laser*

“…( 1) is 300.24 (Pa•cm)/W. [5,6] By comparison, the photoacoustic sensor effectively enhances the value of the pool constant from 300.24 (Pa•cm)/W to 1450.64 (Pa•cm)/W, corresponding to a gain factor of 4.8. The signal sensitivity of the traditional photoacoustic sensor is 0.016 µV/ppb.…”

“…[1,2] As an important detection technique for tracing gas, this technique is widely applied to atmospheric science. [1,[3][4][5][6][7][8][9][10][11][12][13] When there is no problem of saturation, the response of photoacoustic spectroscopy is proportional to the excited laser power, and thus the photoacoustic performance can be improved by increasing the excited laser power. The commonly used methods mainly include two approaches: (i) a laser with higher power is used to increase the energy relaxation process.…”

Optimization design of sphere‐tube‐coupled photoacoustic cell and application of nitrogen dioxide detection

“…Yin et al constructed a differential H‐type photoacoustic cell, which amplified the photoacoustic signal of NO 2 while suppressing the same frequency background noise 18 . Jin et al used a 405 nm low‐power blue light diode to build a system capable of detecting ppb‐level NO 2 , and the response capacity of the system was 0.016 µV/ppb 19 . The following year, Jin et al used the mirror to increase the number of light beam reflections to enhance the photoacoustic effect, so that the response capacity was improved to 0.2562 µV/ppb 20 .…”

“…18 Jin et al used a 405 nm lowpower blue light diode to build a system capable of detecting ppb-level NO 2 , and the response capacity of the system was 0.016 µV/ppb. 19 The following year, Jin et al used the mirror to increase the number of light beam reflections to enhance the photoacoustic effect, so that the response capacity was improved to 0.2562 µV/ppb. 20 Li et al used a 450 nm laser diode as the radiation source, and the laser was reflected multiple times in an absorption cell made of a diffuse reflection sphere, which improved the response capacity of NO 2 to 0.52546 mV/ppm.…”